Disk drive systems that employ accessing heads require that a selected read/write head be transported to a selected data track in a minimum amount of time. To achieve this objective, it is necessary to obtain accurate velocity information so that the velocity of the head actuator may be properly controlled during a SEEK operation. One type of velocity measurement device is sometimes designated as an electronic tachometer, such as described in U.S. Pat. No. 3,820,712, issued to R. K. Oswald, and assigned to the same assignee. However, as track density increases, requirements for accuracy of velocity sensing become more stringent.
A signal commonly used for derivation of actuator velocity is the displacement signal PES. The PES is a cyclically reversing signal passing through zero at each track center and becoming nonlinear in the region of slope reversal midway between track centers. In some prior art electronic tachometers, PES is sensed and differentiated over a given displacement to provide a first velocity signal indicative of the velocity of the accessing mechanism, which may be a voice coil motor, for example. The current in the voice coil motor is detected and integrated over that displacement to provide a second velocity signal that is combined with the first velocity signal to compensate for the discontinuities and nonlinear portions of the PES signal. The combined velocity signal is continuously measured and the accessing device is accelerated or decelerated so that the actual velocity approaches a predetermined ideal velocity.
Some prior art electronic tachometers employ differentiators with wide bandwidths to recover from PES discontinuities. However, systems using differentiators with wide bandwidths tend to suffer from undue noise and nonlinear output at low velocities, among other things. One method for eliminating velocity noise error is to band limit the differentiator. However this approach produces velocity errors at high velocities and accelerations, and excessive phase shift in the velocity signal.